personally, I don't see any advantage why Google should be allowed to index your images. I prefer that Google indexes the pages wherein the images are shown, instead of only the images without further text.
Just remember that Google is not going to report this problem immediatly as solved because you changed robot.txt a halfour ago. Sometimes it takes a month before the google crawler revisits your pages. Only then will Google say.... "Owwww... coppo gave me an new folder to crawl... yummie" and then the crawler will analyse the new folder and add it to your indexed pages.
But again... maybe it's better to let Google only crawl your pages wherein the images reside.